The present invention is directed to a method for bonding materials together, and more particularly to a method for interstitial solid/solid diffusion of a noble metal into a ternary alloy in order to bond piezoelectric ceramics.
Some materials, such as certain crystals and ceramics, exhibit what is called piezoelectric effect, i.e., mechanical strain due to the application of an electric field to the material. In order to take advantage of this effect, several ceramics may be joined together to form a "stack" with means for connecting an electric circuit. More particularly, piezoelectric assemblies may be formed by bonding a stack of electromechanically responsive ceramic elements with electrodes interposed therebetween.
The resulting piezoelectric assemblies may be used for, e.g., frequency control, frequency measurement, electric wave filtering, or interconversion of electric waves and elastic waves. An example of such a piezoelectric assembly is a piezoelectric transducer.
The present invention is directed to a method of efficiently and effectively bonding piezoelectric ceramics together to form a piezoelectric transducer.
Organic chemical compounds have been used to bond ceramic elements and their electrodes into a piezoelectric assembly. Organic adhesives, such as epoxy resins, however, do not possess optimum elastic moduli or electrical conductivity characteristics. For example, epoxy adhesives absorb a large portion of the acoustic energy and provide a low mechanical/electrical coupling efficiency. In addition, the elastic properties of the epoxy change with aging and moisture content. Furthermore, many commonly employed adhesives require relatively high curing temperatures which are harmful to ceramics.
Alternatively, it is known that intermetallic "diffusion bonding" processes provide strong metallic bonds between selected metals. Diffusion bonding is basically the process of facing two elements together under certain temperature, pressure and time conditions, wherein a bond is produced between the facing surfaces thereof due to the diffusion of the elemental components at the facing surfaces. In some diffusion bonding processes, an intermediate layer, e.g., an alloy, is applied between the facing surfaces to effect a bond.
Conventional diffusion bonding processes, however, have several drawbacks. Firstly, diffusion bonding processes using "substitutional" diffusion (in which atoms of different types simply exchange places in the crystal lattice), like adhesive bonding, also require quite high temperatures--generally above the Curie point of ceramics used in piezoelectric transducers--which renders them useless for the production of relatively sensitive transducer ceramics. Secondly, diffusion bonding processes using "interstitial" diffusion (in which smaller atomic species move across the interface to occupy a location in the interstices of the lattice structure of the larger species) generally create very brittle alloys in the bonding region.
More particularly, it is known that indium, either in a pure state or as part of an alloy, and silver may be used in intermetallic bonding. For example, U.S. Pat. No. 2,709,147, issued to Ziegler, utilizes vapor deposited pure indium as a joining metal between a piezoelectric quartz crystal and a body of fused silica. However, this process requires an undesirably elaborate degree of preparation. For example, the flatness of the silica bodies must be very exacting optically, and a very high vacuum condition must be present during vapor deposition. In addition, expensive and complicated facilities are required for controlling vapor deposition and baking. Finally, a relatively high amount of silica/indium intermetallic formation occurs causing poor tensile properties.
U.S. Pat. No. 3,153,839, issued to Pakswer et al., discloses a method for forming a vacuum-tight seal between two glass components using an indium alloy (50% In and 50% Sn) which is heated at a temperature below 200.degree. C. This process, however, provides only a medium magnitude of alloy tensile strength. If this process were used with silver coated piezoelectric ceramics, there would result a high percentage of brittle silver/indium intermetallic phase formations.
U.S. Pat. No. 3,235,943, issued to Marafioti, teaches a method of producing a delay line utilizing a first layer of pure indium soldered at 180.degree. C., a second layer of an indium alloy (98% In, 1% Sn and 1% Pb) soldered at 180.degree. C. and finally, a third layer of an indium alloy (58% In, 41% Sn and 1% Pb) soldered at 135.degree. C. The rubbed soldered joint of pure indium, however, is very weak and displays unreliable joint integrity. In addition, the relatively short time available for soldering is impracticable in regard to bonding stacks of ceramics.
U.S. Pat. No. 3,252,722, issued to Allen, utilizes a relatively thick (2000.degree.A) vapor deposited gold coat and a maximum 0.5 mil. vapor deposited indium coat for joining quartz crystals (nickel and aluminum coatings are also used and serve as barriers in the substrate). Although the temperature and pressure conditions of this method are relatively low, intermetallic compound formations are relatively high, the very thin gold coat is easily dissolved by the thicker indium coat, the concentration gradient is very steep due to diffusion and the tensile property of the indium bulk layer is weak.
U.S. Pat. No. 3,883,946, issued to Dale, discloses a method for securing a semiconductor body to a substrate by applying therebetween an intermediate malleable metal layer of soft solder material having a melting point in the range of 125.degree. to 330.degree. C. This intermediate layer is preferably a lead alloy (95% Pb, 3.5% Ag and 1.5% Sn). A mechanical bond is obtained via the intermediate layer by placing the assembly of the semiconductor body, intermediate metal layer and substrate in a press under a high pressure range of 1-5 tons per square inch and a temperature range of 75.degree. to 300.degree. C. (which is not only below the melting point of the intermediate metal layer, but which is also below the temperature at which any liquid phase would form) for a period of not more than 30 seconds. The resulting bond strength, however, is relatively low.
Finally, in regard to using silver to bond, U.S. Pat. No. 3,448,503, issued to Trott et al., utilizes a silver/silver-amalgam joint under relatively low temperature conditions. However, 100% intermetallic compound formation occurs causing a very brittle joint. In addition, the thin silver electrode dissolves easily during the amalgamation reaction causing poor reliability and the process is very dangerous to perform due to the poisonous mercury vapor created during the process.
From the foregoing, it can be seen that a fluxless diffusion bonding method is desired which produces a bond of high strength and low compliance (i.e., high stiffness), while using relatively low temperature and pressure conditions.